A circuit board on which electrically conductive paste is screen printed is proposed in U.S. Pat. No. 4,801,489 (JP-B-6-34472). The circuit board includes a printed circuit board layer, an insulating layer, and an electromagnetic interference sealed layer. The printed circuit board layer includes an insulating substrate on which circuit patterns are printed. The insulating layer and the sealed layer cover the printed circuit patterns and the insulating layer, respectively. The sealed layer is formed by screen printing conductive paste on top of the insulating layer, and connected with a ground electrode portion.
With this configuration, distributed capacitance is produced between the circuit patterns and the sealed layer rather than between the adjacent circuit patterns. Since the sealed layer is connected with a ground electrode portion, spurious radiation can be reduced.
When the sealed layer is simply screen printed, a film thickness of the layer around the center is thinner than that around outer edges due to a surface tension as shown in FIG. 9. As a result, the conductivity of the sealed layer is unstable. To solve this problem, a larger amount of the conductive paste may be applied to increase an overall thickness of the layer. However, there is a limit to the amount of the conductive paste that is squeezed out by a squeegee in one stroke. Therefore, it is required to repeat steps for screen printing the conductive paste when applying the conductive paste more than the limit.
Moreover, the same problem occurs when screen printing connecting lines such as jumper lines. In this case, electrical resistance of the lines is difficult to estimate, and it affects circuit designing. Variations in the film thickness are also caused by the moving direction of the squeegee during screen printing. This even increases inconvenience in the circuit designing.
The relationship between the moving direction of the squeegee during screen printing and the variations in the film thickness is discussed referring to FIG. 9A. When the direction in which a connecting line extends is parallel to the longitudinal direction of the squeegee set in a position for the screen printing, which is the lateral direction of FIG. 9A, a gap between screen masks 4 is wide. As a result, a screen 3 is largely sagged by a squeegee 2 and a clearance between the screen 3 and a printing surface becomes small. A large amount of conductive paste is squeezed out of an insulating substrate 12 by the squeegee 2, and therefore the line is formed in a small thickness.